FAILURE MODE CONTROL OF X-BRACED FRAMES UNDER SEISMIC ACTIONS

In this article, a new method for designing concentrically braced steel frames is presented. The aim of the proposed method is the design of concentrically braced steel frames able to guarantee, under seismic horizontal forces, a collapse mechanism of global type. This result is of great importance in the seismic design of structures, because local failure modes give rise to a worsening of the energy dissipation capacity of structures and, therefore, to an higher probability of failure during severe earthquakes. With reference to the examined structural typology, the global mechanism is characterized by the yielding of bracing diagonals of all the stories. The proposed method is rigorously based on “capacity design” which requires that non dissipative zones have to be designed to withstand the internal actions coming from the seismic design horizontal forces and the vertical loads acting in the seismic load combination, while non dissipative zones have to be designed considering the maximum internal actions that dissipative zones, yielded and strain-hardened, are able to transmit [Mazzolani and Piluso, 1996]. The new design issue covered by the proposed design procedure is the need to account for the contributions of the compressed diagonals in deriving the design axial force of non dissipative members (beams and columns). The seismic inelastic response of a sample structure is investigated by means of nonlinear dynamic analyses, which have been carried out by means of the PC-ANSR [Maison, 1992] program, where the cyclic behavior of brace elements has been accurately modeled by means of the Georgescu model [Georgescu et al., 1991]. The results of dynamic inelastic analyses carried out with reference to the braced frame designed according to the proposed procedure are compared with those obtained with reference to the same structural scheme designed according to Eurocode 8 [CEN, 2000; CEN, 2003]. In particular, it is pointed out that the application of Eurocode 8 design criteria leads to premature buckling of columns, so that the proposal of a new design approach is fully justified